Re: [PATCH v2 5/5] tcp: Remove mmap_lock fallback path

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/11/26 01:04, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Previously, the per-VMA locking could fail in the face of writers
> which necessitates a fallback to mmap_lock. The new
> lock_vma_under_rcu_wait() will wait for writers instead of failing.

vma_start_read_unlocked()

> 
> Use the new helper. Wait for writers. Remove the fallback to mmap_lock.
> 
> This really is a nice cleanup.

[obama_medal_meme.jpg] ;)

But yeah, it is!

> It removes the need to pass the lock
> state back and forth to find_tcp_vma().
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Acked-by: Lorenzo Stoakes <ljs@kernel.org>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: linux-mm@kvack.org
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arve Hjønnevåg <arve@android.com>
> Cc: Todd Kjos <tkjos@android.com>
> Cc: Christian Brauner <christian@brauner.io>
> Cc: Carlos Llamas <cmllamas@google.com>
> Cc: Alice Ryhl <aliceryhl@google.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: netdev@vger.kernel.org
> ---
> 
>  b/net/ipv4/tcp.c |   31 +++++++++----------------------
>  1 file changed, 9 insertions(+), 22 deletions(-)
> 
> diff -puN net/ipv4/tcp.c~ipv4-tcp-vma-waiter net/ipv4/tcp.c
> --- a/net/ipv4/tcp.c~ipv4-tcp-vma-waiter	2026-06-10 15:57:56.972472379 -0700
> +++ b/net/ipv4/tcp.c	2026-06-10 15:57:56.976472521 -0700
> @@ -2171,27 +2171,18 @@ static void tcp_zc_finalize_rx_tstamp(st
>  }
>  
>  static struct vm_area_struct *find_tcp_vma(struct mm_struct *mm,
> -					   unsigned long address,
> -					   bool *mmap_locked)
> +					   unsigned long address)
>  {
> -	struct vm_area_struct *vma = lock_vma_under_rcu(mm, address);
> +	struct vm_area_struct *vma = vma_start_read_unlocked(mm, address);
>  
> -	if (vma) {
> -		if (vma->vm_ops != &tcp_vm_ops) {
> -			vma_end_read(vma);
> -			return NULL;
> -		}
> -		*mmap_locked = false;
> -		return vma;
> -	}
> +	if (!vma)
> +		return NULL;
>  
> -	mmap_read_lock(mm);
> -	vma = vma_lookup(mm, address);
> -	if (!vma || vma->vm_ops != &tcp_vm_ops) {
> -		mmap_read_unlock(mm);
> +	if (vma->vm_ops != &tcp_vm_ops) {
> +		vma_end_read(vma);
>  		return NULL;
>  	}
> -	*mmap_locked = true;
> +
>  	return vma;
>  }
>  
> @@ -2212,7 +2203,6 @@ static int tcp_zerocopy_receive(struct s
>  	u32 seq = tp->copied_seq;
>  	u32 total_bytes_to_map;
>  	int inq = tcp_inq(sk);
> -	bool mmap_locked;
>  	int ret;
>  
>  	zc->copybuf_len = 0;
> @@ -2237,7 +2227,7 @@ static int tcp_zerocopy_receive(struct s
>  		return 0;
>  	}
>  
> -	vma = find_tcp_vma(current->mm, address, &mmap_locked);
> +	vma = find_tcp_vma(current->mm, address);
>  	if (!vma)
>  		return -EINVAL;
>  
> @@ -2319,10 +2309,7 @@ static int tcp_zerocopy_receive(struct s
>  						   zc, total_bytes_to_map);
>  	}
>  out:
> -	if (mmap_locked)
> -		mmap_read_unlock(current->mm);
> -	else
> -		vma_end_read(vma);
> +	vma_end_read(vma);
>  	/* Try to copy straggler data. */
>  	if (!ret)
>  		copylen = tcp_zc_handle_leftover(zc, sk, skb, &seq, copybuf_len, tss);
> _